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Running the following code:
>>> import msilib
>>> db = msilib.OpenDatabase('c:/windows/installer/1c7a1.msi', 1)
>>> db
<_msi.Database object at 0x01E230A0>
>>> dir(db)
(Python crashes - tested on current Trunk and Python 2.7.1).
I tried tracking it through the C code - and it seems to be a problem when dir is checking for __dir__ (the pointer in PyObject_GetAttrString seems to be incorrect).

A call to PyType_Ready() fixes the issue, see attached patch.
I added a test that opens a random file in c:\windows\installer, I don't know if this is correct for all Windows version.

That fixed it - but it seems we need that for the other Types defined in the module.
Regarding testing - would it be a good idea to add an MSI to the test suite - or better to create one during testing (using msilib) and then use that in the tests?

If we can generate a testable MSI file that would be the best. Including a very small pre-generated MSI for the purposes of the test would be acceptable.
As-is, the tests don't pass because my machine has C:\Windows\installer1032円f.msi that gets used for the test, which apparently doesn't work with this functionality, and other machines may end up with the same situation. I'm not sure if that's a bug in the code or in whatever MSI that is, though. The OpenDatabase call returns "unknown error 6e" for both new tests.

I wasn't so happy trawling through \windows\installer either :) 
Creating an MSI to test is very simple, and actually quicker than I had originally thought. The latest patch (support_dir_for_msi_objs.patch) creates the one and just uses that. Optionally I could create a new one each time setUp()/tearDown(). But was thinking - let's do that if/when we need to :)
The latest patch updates the tests - but no change to the C code.
(I doubt the error you received is a bug in msilib, and I couldn't repro even by opening EVERY MSI in my windows\installer dir. It could be a rights/security issue? The previous test was not opening as READ ONLY - even though it never saved the db, so that may be another reason?)

The attached patch is short and sweet and looks okay to my untrained eye. I believe it should apply cleanly as there've been very few changes to _msi.c. Can we have a formal patch review please.

This is a generic issue as there are multiple unready types that can raise (or in this case crash) with code that should work. #26906 gives other examples and discussion of possible generic solutions that would make this issue obsolete.